Intraocular tumors have been treated in the last 30 years using proton therapy with outstanding clinical results.
A proton beam delivered through a fixed horizontal beam line is targeted to the patient lesion to damage the DNA of the tumor cells and block their proliferation.
During ocular protein therapy the patient is seated on a robotic chair and immobilized using a thermoplastic mask and a bite block.
Treatment is divided into three phases: simulation, planning and delivery.
During the simulation phase the diseased eye is roughly aligned to the beam axis by movements of the robotic chair.
Then, a couple of radiographies are required using the in-room orthogonal x-ray imaging system while the patient is looking in a straightforward direction.
These images are used to identify the position of tantalum clips that are sutured to the outer surface of the eye in a pre-treatment surgical procedure.
The treatment planning system I plan, builds a geometrical eye model that includes tumor, eye globe, lens
and other notable ocular structures upon the clips configuration measured at time of simulation.
Then treatment planning is done.
This procedure consists in finding the best geometrical configuration in terms of gaze direction
to achieve complete tumor coverage and optimal radiation sparing of the surrounding healthy tissues.
Finally, during treatment delivery the patient is asked to fixate a light prepositioned according to the treatment plan prescriptions.
In this phase the patient wears eyelid retractors allowing for an undisturbed entrance pathway of the proton beam.
An eye tracking system was integrated in the beam line. It consists of four optical cameras positioned in pairs symmetrically to the beam axis.
Two on the left side and two on the right side.
This configuration was adopted to guarantee sufficient high visibility by at least two cameras regardless of the patient gaze direction.
The system was geometrically calibrated with respect to the beam axis and cross calibrated to the x-ray imaging system using an ad-hoc phantom.
As a result of this calibration procedure the eye tracking system can recognize the three-dimensional position of the pupil with respect to the beam Iso-center.
One can assess the geometrical transformation required to navigate the eye from simulation to treatment position
and apply it to find the pupil position corresponding to the planned gaze direction.
As a result by comparing the real-time measurement of pupil position with this reference the ETS provides on a patient-specific basis an estimate of the correction,
in form of chair translations required to align the eye to the planned treatment position.
After this pre-alignment x-ray imaging of clips was performed to fine-tune these corrections.
We compared the tumor beam alignment accuracy achieved with these two methods.
A median three-dimensional clips misalignment of 0.42 millimeters and 0.62 millimeters was measured with x-ray and pupil-based navigation respectively.
Considering the tumor volume center of mass the median errors achieved with the ETS was 0.49 millimeters compared to 0.32 millimeters for x-rays.
Results show that the pupil-based approach does not guarantee the same level of accuracy of the clinical standard based on clips.
This is a direct consequence of the simplified eye tracking method applied which does not account for eye rotation.
However conventional eye tracking methods based on corneal reflections could not be applied.
Eyelid retractors prevents blinking and the resulting eye surface dryness compromises corneal reflections visibility. Alternatives are currently under investigation.
In addition we quantified the discrepancies between the ETS measurement of the pupil and its absolute position on the geometrical eye model.
Even though we measured differences below 2.2 millimeters a clear patient-specific bias could be observed.